# Understanding the Periodic Table of Elements

First developed by Russian schoolteacher Viktor Mendeleev, the Periodic Table (a.k.a. Mendeleev’s Table) is a chart that shows the elements arranged by atomic number and chemical properties.

The periodic table (picture here) is a grid, 18 blocks across and 7 blocks high. There are a number of blank spaces – most notably, the 16 empty spaces between Hydrogen and Helium. In each space that contains an element, any standard periodic table will list the following:

– Atomic Symbol: H, O, Hg, are all examples of Atomic Symbols. These are located in the middle of the square.

– Atomic Number: Located immediately above the Atomic Symbol, Atomic Number is an integer (a whole number, such as 1, 2, 3, 4 etc.) and it refers to the number of protons in that atom.

– Atomic Mass: The location of Atomic Mass is not standardized, but it is distinctive because it is a decimal. It represents the average mass of all the isotopes, weighted by the frequency with which each isotope occurs. For example, if 99% of Oxygen atoms weigh 16 AMU (Atomic Mass Units), and 1% weigh 17 AMU, then the Atomic Mass will be written as .99*16+.01*17=16.01 AMU. Caveat: Sometimes the Atomic Mass will be an integer written in parentheses, e.g. (277). This means that the element is artificial, and that only one isotope can be created.

– Atomic Name: This is a whole word, such as Oxygen, Lithium, or Seaborgium, and it is usually written immediately beneath the Atomic Symbol. The names of some atoms start with “Unun”, and this means that they have not been formally assigned a name by IUPAC, the international body that assigns names.

The periodic table was cleverly designed so that any column (series of atoms stacked on top of one another) consists of elements with similar properties. The first column on the left-hand side are known as Alkali Metals, and have in common their tendency to give up one electron during chemical reactions. The second column, the Alkaline Earth Metals, tend to give up two electrons. The transition metals columns three to twelve, and have in common mostly that they are (a) what we think of as being metals and (b) utterly bizarre in their general behavior.

The final column is the “Noble Gases“, a group of gases distinguished both by the fact that their names all end in “on” (Neon, Xenon, Krypton, Argon) and their refusal to react with any other chemicals, which is thought of as being similar to nobles who refuse to interact with “common elements.” The second-to-last column is the Halogens, all of which tend to give up one electron in reactions. The third-to-last column is the Chalcogens, which give up two electrons in reactions.

If you have a picture of the periodic table out in front of you, this is the part where you start wondering “but what about that block that’s just floating off in space, at the bottom”. Well, those are the Rare Earth Metals. The upper row is the Lanthanide Series, and the lower row is the Actinide Series. Neither is particularly important in modern chemistry, because these elements exist only in nuclear reactors, and nuclear bombs. And why they’re floating off at the bottom? Well, if you put them into the periodic table, it would be too wide to fit on one page. Chemists are practical people, after all!